Stereo amp idea

[Auke Jolman]

Hi All,

The attached schematic is a stereo amp of sorts. The idea is to take the signal after the MV and send it to the main poweramp and to a stereo pedal. The stereo output off the pedal is then fed into 2 separate poweramps.

Would this be possible and/or are there improvements I can/must apply?

Thank you in advance for your comments and remarks.

[jjasilli]

There's lots of schematics on the Forum that show how to do FX send & return loops.  Your cathode driver idea is basically good, but TS should not come between it and the FX out.  That's because the TS attenuates signal voltage and messes with output impedance.  You probably want a boost or buffer on the FX in side, with a voltage control pot.


Even though you say you want stereo, you actually wind up with 3 power amps.  Not sure what this purpose is.


Also, I have not checked out the preamp, power amps or PS.  Just focusing on the FX aspects.

[sluckey]

Most pedals expect an instrument signal level input and put out an instrument level signal. Probably won't drive an EL34 directly, especially if the pedal only has a single 9V battery.

[Auke Jolman]

Thanks for your replies.

@ jjasilli:

I will look into the buffer. Indeed I've seen several passing by.

My idea is to have 1 poweramp with the dry signal, and feed the 2 other poweramps with the stereo output from say a Boss DD7. I've attached a block schematic of my idea.

@ Sluckey:
Do I need to put an extra gainstage in there to drive the EL34 and/of can that be done by using a buffered fx-loop?

[shooter]

Do I need to put an extra gainstage

you've got enough gain stages to drive a 1000w linear power tube
Like JJ said, put the TS further left and use 1/2 a XX7 as TS recovery
I'd also up the bias pot to 5W just in case
also mentioned there's a lot of info on FX, study them

my kt88 build I'm running about max dissipation, it wants something close to 35Vac to drive it into distortion, with Plate  ~ 400vdc with Ip ~~ 100mA.  The point, start there, it's only 1 tube and a handful of parts, then you have a better Idea of what you need left of that.

[Auke Jolman]

Hi All,

I added a tube recovery for both the FX-returns.

Please feel free to comment on the changes. I'm looking forward to your responses.

[jjasilli]

Some comments,


Once again, the FX send should not come after the TS.  It should be driven directly off its own, dedicated cathode follower.


The 100K input impedance of the FX return seems low???


The stereo power tubes need a grid leak resistor.

[Auke Jolman]

Hi JJasilli,

Thanks for your comments.

I changed the schematic following your comments. I'm not quit sure about the placement of the cathode follower and implementation of it. Hopefully you or anybody else can chime in again.

[pdf64]

Your CF for the fx send won't have much signal swing; it would be better for the cathode to be elevated, see http://www.valvewizard.co.uk/accf.html
Why take its feed from before the tone controls?

[Auke Jolman]

Hi All,

Thanks again for the replies.

I must say I'm confused as to what the best way is to take the signal for the fx-send and what buffer I need. Looking at Marshall schematics a likes, it seems that the effects-loop is placed after the tone-stack, hence my confusion with regards to the comments by jjasili and shooter and pdf64.

I added 1M A pot after the recovery stage, so I think that covers the grid leak of the stereo powertubes. Hoping not to make things worse by doing so  . For me this is the first time not to take an existing schematic, at least not completely and turn it into something beautiful.

@ jjasili and shooter:
Would it be possible for you to explain to me why you think it is important to take the fx-send signal before the tonestack?

@pdf64
I read the article you mentioned and changed my schematics accordingly, at least that is what I think I did.

Still trying to stay a float...

[jjasilli]

I must say I'm confused
Yes, your design goals exceed your current level of ability and understanding of circuits.  This is not a criticism; it applies to all of us. One alternative is to start more simply, and plagiarize an existing, proven circuit that you like. Add complexity after you have achieved proven results.  E.g., add an FX loop to an existing amp; or build an amp with an effects loop.  Get that to work. Then try sending signal to another power amp.  All I got was hum. 

[Auke Jolman]

Hi jjasilli,

Thanks for your reply.

I know I'm lacking a lot, but still I would like get this thing on the road and learn something new in the process.

I've build several amps during the last years with and without effects loops, both passive and buffered + recovery stage. Whenever I had some problems with getting them to work, I could turn to you guys. Your replies and comments have always helped me a lot and got the amps working eventually.

For me it is not a problem to play in stereo with the number of amps I have. I've added a passive line out to some of them. That amp is my dry signal. The line-out then is used to feed a stereo pedal that feeds 2 other amps. And like you, I got instant massive hum indeed along with it. I found that a quick solution was to only ground 1 amp and let the other 2 amps use the ground of the grounded one. Later I've build an isolation pedal that uses a little transformer for the isolation part. Feels a lot better with regards to the grounding issue. That all works great. But, I've always got to turn on more than 1 amp and then my wife comes in and says "Why do you use that many amps! ". So I thought that could be fixed by incorporating it all in 1 amp. Hence my effort.

For me building an amp is a great hobby. When I read some posts I'm sometimes blown away with the knowledge a lot of you guys have. Still hoping that some of that knowledge can be spared to get my schematic in line.

[sluckey]

Your 12AU7 cathode follower will have big voltage on pin 2 and 3. You will need coupling caps on the input and output.

[2deaf]

Striving for minimum.  Yeah, I know that one EL34 could have as much as a 1M grid leak, but with the gain in the pre amp that pot would usually be turned down.

[Auke Jolman]

Thanks SLuckey and 2deaf for your replies.

@ Sluckey:
I'm glad you saw the omission with the coupling caps.

@ 2deaf:
Your schematic is more in the line of my initial plan with some clever additions. I will work it out in more detail (expected voltages and getting all the labels right and so fort). I will also try and put a layout together based on your altered schematic.

[shooter]

If you haven't started throwing solder yet, 2Deaf's looks good.  there are a couple amps I studied before I tried a stereo build, Ampeg's SET is a great mind bender to study

[Auke Jolman]

Hi Shooter,

Thanks for your reply.

I too thought that is is something to pursuit further. I plan on working on the schematic this evening. When I have it finished I will upload it again.

[Auke Jolman]

Hi All,

I've finished the updated schematic.

I'm planning on doing a complete layout. When I've got it finished, I will come back with that.

[Auke Jolman]

Hi All,

Here's an update of the paperwork.

I've put a layout together and made some minor changes to the schematic.

If the schematic and layout don't match, it's probably wise to go with the schematic.

Feel free to comment on the layout and schematic.